1. Field of the Invention
The present invention is directed to a method for measuring the field pattern of electrical or magnetic fields using a sensor arrangement, as well as with a data processing system for iterative calculation of the spatial position of a biological power source which causes the field.
2. Description of the Prior Art
The measurement of magnetic fields which are weaker by several powers of ten in comparison to the earth's magnetic field is necessary for use in measuring biomagnetic activities. This requires measuring external fields emanating from nerve currents in the inside of a living organism using sensors at various locations, and calculating a dipole equivalent to the location of the source based on the field pattern measured by sensors, and with a corresponding data evaluation program. The measuring instruments required for this purpose must be extraordinarily sensitive, and special gradiometers are employed for these purposes, contained under superconducting conditions in a Dewar vessel together with a superconducting quantum interferometer device (SQUID). In order to make a meaningful measurement for this manner, the external noise fields must be shielded in the best manner possible. To that end, the patient and the measuring apparatus are accommodated in a walk-in magnetically shielded chamber. To be able to calculate the field distributions as quickly as possible, multi-channel systems have been developed as disclosed, for example, in the publication "Electromedica," Vol. 57, 1989, pp. 2-7.
It is not only the noise fields which act externally, and falisfy the measured result. It is often the case that the bioelectric power source causing the external magnetic field is additively superimposed with the activities of a neighboring source. This "background activity" can result in an error in the localization of the biological power source in question. This particularly arises in the case of biomagnetic cardiac activities (magnetocardiogram). When such a superimposed field is known with sufficient precision, it can be subtracted from the measured field to obtain the field of the power source in question. Such a constant activity, however, is not present, particularly given the stimulation currents in the heart. Thus, an electro-physiological activity in this region during a known time segment generates an electrical or magnetic field whose spatial field pattern remains approximately constant in this time span, but changes by an amplitude factor dependent on time, for example, slowly increases and decreases. The curve of this chronological change in the amplitude factor is unknown. For example, such a change arises when an electro-physiological activity remains stable in terms of location and direction, but the power density of the ion flux slowly decreases. This occurs, for example, in the repolarization of the cardiac atria in the P-Q interval of the heart cycle. After the beginning of a defined time segment (time window), this activity having a constant field pattern is superimposed on a further activity which is to be localized. This background field pattern can be measured at a defined time preceding the beginning of the activity which is to be localized, and if this activity were to remain constant, could be subtracted from the field pattern to be measured. In the case of the above practical application, however, the background activity does not remain constant during the duration of the measuring cycle to permit the activity to be calculated with sufficient precision, so that the above simple subtraction method cannot be used.